The present invention relates to polyurethane pads and methods of producing the same.
The present invention generally relates to polishing pads, in particular to chemical-mechanical polishing (CMP) pads. CMP is a process step in the semiconductor fabrication sequence that has generally become an integral part of the manufacture of semiconductor wafers. The process is used in a variety of applications in the semiconductor fabrication sequence. Different applications are sometimes best optimized with different polishing pad types and configurations, however, this invention is not application specific.
In any of these CMP processes, the silicon substrate is forcibly placed in direct contact with a moving polishing pad. A wafer carrier applies pressure against the backside of the substrate, usually while simultaneously forcibly applying rotation. During this process a slurry is made available, and is generally carried between the wafer and the pad by the motion of the pad. The composition of the slurry is dictated by the specific application.
The CMP polishing pad is required to perform a plurality of engineering functions. It is required to polish, planarize up to a certain planarization length (L) determined by the quality of the silicon substrate, not planarize beyond that length, transport slurry, maintain the same friction with the wafer for wafers polished sequentially and with interruptions for hundreds of wafers, clean the wafer surface, not scratch the wafer surface, be replaceable in minimal time, and others. This invention addresses the planarization length of a pad.
L is defined as a lateral dimension characteristic of the pad's ability to planarize. Intrinsic to this concept is Preston's equation, which maintains that when polishing, the removal rate is proportional to force. There are significant deviations to this relationship, but it holds generally, and for our purpose, it is sufficient. With Preston in mind, one can consider a feature to be planarized consisting of an upraised element. (FIG. 1) A polishing pad will try to planarize the feature, and will succeed in doing so when the pressure exerted by the pad at the top of the feature exceeds the pressure exerted adjacent to the feature. Ala Preston, the removal rate at the top of the feature will exceed the removal rate adjacent to the feature and overtime the feature will decrease in height. One definition of planarization length is the distance from the feature that the pressure has increased to 1/e of the pressure infinitely far from the feature (e is ln(10)). Other definitions will also suffice for this discussion.
Both the silicon wafer and the platen offer their own sense of planarity. Silicon wafers generally exhibit small but nonzero undulations in their thickness. These undulations are on the order of 300 A and are spaced apart by a distance on the order of one centimeter. These undulations are completely unrelated to the surface features of the wafer and planarizing them generally leads to more material being removed from some spots than from other spots, an undesirable effect (FIG. 2). Additionally, it is also not desired to impose any deviations from flatness exhibited by the platen onto the wafer. Therefore, the requirement on the polishing pad related to L is that L must be as long as possible but substantially less than 1 cm. This control of L is typically achieved by use of a stacked pad, where the top pad does the polishing and the planarizing, and the bottom pad serves to decouple the platen from the top pad, and allow enough flexure to render as insignificant, pressure differences due to topography variations on the order of 300 A over a distance of 1 cm. Stacked pads have been described in U.S. Pat. No. 5,257,478 by Hyde, U.S. Pat. No. 5,212,910 by Breivogel, U.S. Pat. No. 5,287,663 by Pierce and U.S. Pat. No. 6,362,107 by Shiro.
A bottom pad typically utilized to achieve these results is the Suba IV from Rodel. Such a pad is very soft and serves the purpose of decoupling the platen from the upper pad. As suggested by U.S. Pat. No. 5,257,478 by Hyde, et al., an ideal value for this bottom pad is <250 psi. Further, the bottom pad should be resilient or elastic. A schematic diagram indicates the subpad achieving this objective. (FIG. 3).
An additional engineering requirement of the base pad is that it remain elastic throughout its life, and uniformly elastic at least everywhere under the wafer track. For example, since water is a natural component of all CMP processes, both the top and bottom pad must not change their flexural modulus in the presence of water. Additionally, since the CMP process involves the use of pressure, the pads must not exhibit a significantly inelastic component. Under pressure, an inelastic material will undergo permanent deformation. If either the subpad or the top pad undergo significant permanent deformation, a degradation of the performance of the pad could result. This could happen for a plurality of reasons of which we consider two: 1) A permanent deformation can change the elastic modulus of the material, putting it in an undesirable range; and 2) a nonuniform change in the elastic modulus can change the uniformity of the removal on a wafer, also highly undesirable. Therefore the pad must exhibit primarily elastic behavior.
So while the sublayer performs a supporting function and the top layer performs a working function, their parameters relative to one another have to be selected very carefully to achieve the above identified results. It is believed that the existing multi-layer polyurethane pads can be further improved in this sense.